Project Summary Obesity, which affects one in three Americans, is a major risk factor for hyperlipidemia, hypertension, insulin resistance, type 2 diabetes (T2D), cardiovascular disease, and certain types of cancer. Recent studies have suggested that alterations in the gut microbiota may be a major contributor to this increased prevalence of obesity, however the mechanism by which this occurs is unknown. The Overarching Hypothesis guiding the aims of this grant, based on our recent studies, is that a high-fat diet (HFD) increases net production of acetate by the gut microbiota, leading to activation of the parasympathetic nervous system, which in turn promotes increased glucose-stimulated insulin secretion (GSIS), increased ghrelin secretion, hyperphagia, obesity and its related sequelae of hypertriglyceridemia, non-alcoholic fatty liver disease (NAFLD) and insulin resistance. These findings open the door to new therapeutic interventions, but the current lack of understanding of the underlying mechanisms, risk factors, and effect of existing T2D drugs is a significant roadblock. In this proposal, we present a plan to first determine how HFD increases net acetate production by the microbiota. Second, we will assess whether human gut microbiomes, which vary broadly between individuals, are differentially susceptible or resistant to HFD-induced acetate production. Finally, we will determine whether changes in the microbiota with metformin therapy may contribute to its efficacy for the control of hyperglycemia in T2DM patients. Specifically, we will test the hypothesis that metformin causes changes in the intestinal microbiome that contribute to reduced postprandial hyperglycemia via altered acetate production.